Volume 2: Assessment of Environmental Effects 296 14 Geotechnical Hazards Overview This Chapter provides an assessment of the underlying geotechnical conditions to identify: any potential liquefaction subsidence; and any potential settlement effects from artesian groundwater pressures. The location of the bridge has high seismicity and is within close proximity to the Wellington Fault.The Project Area is typical of the geology of the Wellington region where historically (pre 1855) the Basin Reserve was a low-lying swamp. Therefore the geology of the Project Area comprises deep layers of fill material and alluvium and groundwater is at close to the surface in the lowest parts of the Project Area. The loose to medium dense alluvial soils give rise to the primary geotechnical issue for the Project, which is potential liquefaction of sections of the upper soil profile (9m) during an earthquake event. Site investigations of the ground profile and consideration of elevated water pressure conditions have been undertaken. The results identify that there is a potential for liquefaction in a design earthquake event of a 1 in 2,500 year event. There is potential in this scale of event that subsidence from liquefaction would cause settlement of the pile head and thence damage to surrounding infrastructure. To mitigate these subsidence effects during any earthquake, the underlying ground conditions have been investigated, analysed, and incorporated into the design of the bridge structure to ensure their resilience. The Project has also considered any potential effects of the proposed piles on the groundwater regime, the management of groundwater during construction of the piles, and any actual or potential effects of drawdown and settlement effects on buildings in the vicinity of the proposed piles. The groundwater regime includes artesian groundwater. It is expected that groundwater flows from the piles is likely to be slow and that large groundwater flows are an unlikely scenario. There are a number of pile design and construction techniques that have been incorporated into the construction methodology to avoid and mitigate any actual and potential effects to the groundwater regime. Therefore ground subsidence as a result of drawdown from bridge piling is unlikely. Even allowing for worst case scenarios for long term groundwater pressure reduction from piling, subsidence effects will be no more than minor, and the bridge foundation system has been designed fully taking into account the underlying ground conditions to insure structural integrity of the bridge.
Volume 2: Assessment of Environmental Effects 297 The conditions at the Basin Reserve are not untypical in Wellington and design and construction methods are available that minimise effects associated with liquefaction subsidence and artesian groundwater pressure. Therefore overall, it is considered that subsidence effects would either be avoided or be no more than minor and the structural integrity of the bridge has been designed to fully take into account the underlying geotechnical conditions. 14.1 Introduction This Chapter identifies and assesses the potential geotechnical hazards that are relevant to the structural design and construction of the Project. The information contained in this Chapter summarises information in Technical Report 1: Design Philosophy Statement in Volume 3 of these documents. A critical stage in the structural design of the Project has been to understand underlying geotechnical conditions and having appropriate geotechnical design components for the bridge structure to increase its resilience. The bridge has been designed in accordance with the NZTA Bridge Manual, Second Edition 2003 and where the Bridge Manual and associated material standards do not cover the specific design issues the appropriate Australian Standard or a British version has been used. The design standards require that the bridge structure be designed to perform well (remain substantially intact) in a 1 in 2,500 year event. This performance requirement ensures that the structure is designed with the knowledge of the underlying ground conditions in order to best withstand such an event. The following Chapter outlines the underlying geotechnical conditions, the potential effects of the bridge structure, and the proposed mitigation measures in relation to this. 14.2 Existing geotechnical environment The Basin Reserve was uplifted from a low-lying swap in the earthquake of 1855. However, it remains the lowest point in the locality. It is bounded by Mount Cook to the west; Mount Victoria to the east, Newtown valley to the south and the Te Aro flats to the north. The geology has four layers, starting at the surface, as follows: existing fill, comprising of silty sand and gravels, to depths varying from 1.5 to 3 metres; recent alluvium, comprising interbedded loose to medium dense sandy gravel, silty sand, and firm to stiff silt, clay and gravelly sandy silt, to a depth of 9 metres;
Volume 2: Assessment of Environmental Effects 298 older alluvium, comprising interbedded dense to very dense sandy gravels with some silt; and, Wellington Greywacke, comprised of highly weathered to completely weathered sandstone and mudstone with varying degrees of strength. Site investigations show that at the Terrace Tunnel and the Mount Victoria Tunnel, the layers of fill and alluvium are relatively shallow and the greywacke rock is close to the surface. As you move towards Cambridge and Kent Terraces Project Area from both the Terrace and Mount Victoria tunnels, the layers of fill and alluvium become much wider and extend deeper with the greywacke rock much further below the surface. The Wellington region is an area of high seismicity. The region has a number of major active faults and as such there is a risk significant earthquakes. The Project Area also has the potential for moderate liquefaction. These factors influence the design and extent of the bridge piles. The Project Area has a complex groundwater regime, including sub-artesian and artesian groundwater levels. The groundwater can be characterised into three broad aquifers shallow, middle and deep. Low permeability layers of clay and silt separate the aquifers and it is likely that the aquifers are leaky. The shallow aquifer extends from ground surface to down to depths of 12m, it is unconfined and ground water is generally at ground surface. It is in the Holocene age deposits and Upper Pleistocene age deposits. The middle aquifer is generally between depths of 15m and 25m, it is confined but may not be continuous The ground conditions in this aquifer are typically medium dense to dense silt, sand and gravel, with stiff clay / silt layers of Pleistocene age. The ground water level is artesian with a ground water head between 6 and 9m above ground level. The deep aquifer is located in the ground immediately overlying bedrock and the upper part of the weathered bedrock sequence. The ground conditions are dense gravels, sand and silt, and weathered greywacke bedrock. The ground water head for the aquifer is about 3 to 6m above the ground surface. 14.3 Assessment of geotechnical hazard effects 14.3.1 Liquefaction The key geotechnical issue is liquefaction of the upper soil profile during an earthquake event causing effects to the bridge structure. An assessment was carried out to calculate the potential for liquefaction by analysing the soils and elevated pore pressures in artesian conditions in the Project Area.
Volume 2: Assessment of Environmental Effects 299 The analysis estimated that: the upper soil profile to 9m depth is made up of 25% thin loose lenses of silty sand; the portion of soils that are potentially liquefiable within the top 9m of the soil profile vary considerably from location to location within the site. each liquefiable lens within the upper soil profile is typically less than 1m thick; and sands between 18m and 23m may liquefy in an earthquake. Liquefaction subsidence is predicted to be: in the order of 200mm to 250mm for level ground; and 30mm to 100mm is expected to result from liquefaction of layers in the top 9m of the ground profile. The remainder of the subsidence is expected to occur as a result of liquefaction of the soils between 18m and 23m deep. Any potential subsidence from liquefaction would cause soil above liquefied layers to drag on the bridge piles and may cause settlement of the pile head and damage to surrounding infrastructure and properties. However, these conditions are not untypical and the bridge foundations have been designed to mitigate the effects of ground subsidence in a design earthquake. The piles extend below the liquefiable layers and have sufficient capacity to withstand the effects of liquefaction and consequential down drag. 14.3.2 Groundwater Another geotechnical issue is any potential subsidence of the ground in the surrounding area during the bridge construction. It is considered there would be little interaction with the groundwater regime during construction except for the construction of the bridge piles. In this regard, the geotechnical state and artesian water pressure of the Project Area is relevant to ensure that the ground conditions are adequately addressed. The Project assessment has identified that there are a number of potential effects during construction of the bridge piles given the variable ground water regime. These are: leakage of groundwater along the pile stems resulting in a lowered pressure in the ground water regime in the long term; drawdown of groundwater levels during construction, either due to the interconnection by the pile hole, or due to a deliberate draw down of the groundwater to facilitate construction; and,
Volume 2: Assessment of Environmental Effects 300 ground water pressure and flow for the pile holes leading to flooding and/or difficulties during construction. A significant change in ground water pressure, temporarily or permanently as part of the Project has the potential to cause subsidence and consequently settlement damage to the surrounding infrastructure and buildings. A number of geotechnical bores have been drilled as part of site investigations. From these bores groundwater flows have always been small. The ground water is artesian, with a ground water head or pressure of between 6 and 9m above ground level. With appropriate groundwater management associated with the pile construction (see section 14.4 below) it is considered unlikely that groundwater drawdown, and consequent ground subsidence, would occur. Notwithstanding this conclusion, an assessment has been made of the sensitivity of key buildings in the vicinity to the unlikely event of groundwater drawdown and subsidence. Based on the ground conditions, and assuming a 5m reduction in the groundwater pressures in the middle aquifer and long term groundwater drawdown, then subsidence is estimated to be in the order of: 25 mm at the Grandstand Apartments, and less than 10 mm at the RA Vance Stand, Mitsubishi Motors, the St Joseph s Church and the commercial properties on Ellice Street. Short term groundwater drawdown during construction is likely to give a much smaller subsidence. The level of subsidence assessed under both short and long term ground water drawdown is not expected to cause an angular distortion sufficient to cause damage to the buildings. These assessments are consistent with the minimum subsidence of the ground and associated settlement of the ground adjacent to the trench structure at Wellington Inner City Bypass, where the groundwater was drawn down several metres for 12-18 months during construction. The ground has holocene and pleistocene deposits that are similar to those present in the middle and deep aquifer in the Basin Reserve area. Overall, the effects to groundwater are considered to be no more than minor. 14.4 Measures to avoid, remedy or mitigate actual or potential geotechnical hazard effects It is considered that potential liquefaction effects and potential subsidence effects as a result of groundwater interference should be able to be mitigated through careful and appropriate geotechnical design and construction management.
Volume 2: Assessment of Environmental Effects 301 14.4.1 Bridge piers/piles Piles of appropriate size and resistance to deformation are recommended to provide good performance in earthquake events. Piles are likely to need to extend to the bedrock, and thereby into all three aquifers. The design recommends bored piles in comparison to driven piles. Bored piles: have the ability to be advanced through dense gravels and the shear, crush and fault zones in the bedrock to a suitable depth to resist vertical and lateral loads; and are stiffer and more able to resist deformation. The scheme design recommends: 900mm to 1200mm diameter bored piles founded at depths of 30m to 36m in Greywacke bedrock, or where the depth to Greywacke bedrock is greater than 35m founded in dense to very dense alluvium; temporary or permanent casing will be advanced through the recent/holocene deposits or 7m minimum depth in order to avoid issues of collapse of the hole during construction and during earthquake events. A smaller diameter casing will be used to advance the pile below this depth; bridge piles that are piled with reinforced soil abutments should be sleeved to allow for displacement of the reinforced soil wall or approach fill in earthquakes; to avoid issues with down-drag on piles at the abutments, the construction of the abutment piles is not programmed until settlement of the approaches is mostly complete; reinforced earth walls are formed at abutments using selected fill materials most likely with vertical walls facing; weak cohesive or compressible deposits below the foundation of embankments will be undercut and removed. The undercut to remove any soft and loose materials underneath the approach embankments is estimated to be 1.0 2.0 m based on available information. If greater thicknesses are indicated, then consideration should be given to stabilisation and preloading measures with surcharge, as appropriate; and ground improvement with piles or deep soil mixing is likely to be required at the abutments to alleviate potential liquefaction issues. The detailed design phase may consider alternative pile sizes, and the need for temporary or permanent casing assessment and the related effects of doing so. The above recommended construction methods would be further assessed and finalised during the detailed design and construction phase and managed through the CEMP in Volume 4 of these documents where appropriate.
Volume 2: Assessment of Environmental Effects 302 14.4.2 Groundwater There are a number of possible solutions to manage groundwater during construction. The exact method of construction will be confirmed through detailed design. The short term issues associated with construction could be managed by: temporary steel casing of larger diameter (1800mm) will be installed over the upper 10m depth of the hole into the aquiclude an inner casing will be used to advance the pile beyond this depth with the annulus between the casings and filled with water or bentonite; groundwater pressure instrumentation will be installed within the aquifer layers to monitor the effects of piling on groundwater. These instruments will be monitored to ensure that the temporary groundwater pressure changes are within allowable tolerances; should larger volume of flow from ground water be encountered, there will be contingency measures to extend the casing above ground, and deal with the flows during the short timeframe before the pile is concreted; and tremie placement of concrete (using a pipe to place concrete below the groundwater level) for the pile construction, which is normal standard practice. The potential for long term leakage should be managed by special measures, such as: after the main pile is constructed the annulus (gap or join) between the pile and outer casing will then be grouted and the outer casing removed. This is to provide an effective long term seal against ground water leakage. a contingency measure if leakage occurs would be to inject additional grout into specific areas where groundwater is leaking. (tube-a-manchette techniques). The above measures are proposed to mitigate any potential adverse effects for ground subsidence as a result of interference with groundwater. Whilst the effects are anticipated to be no more than minor with the above measures in place, it is recommended that pre-constructional structural surveys of key buildings and monitoring of settlements and groundwater in the area should be carried out. 14.4.3 Summary The design and construction methodology of the Project recognises the underlying geotechnical conditions. The structural design is responsive to the ground conditions to ensure reliance of the bridge during a design earthquake event.
Volume 2: Assessment of Environmental Effects 303 Notwithstanding the assessment that subsidence can be avoided through construction methodologies, it is proposed to monitor settlements and groundwater levels in the area, and carry out pre-construction building surveys of key buildings and this is reflected in the CEMP and the conditions of consent. Work undertaken to date suggests that any subsidence effects as a result of interference with groundwater are likely to be minor, but monitoring during construction is recommended. Therefore, in terms of constructing the Project and in particular the bridge, there is no reason why geotechnical or groundwater conditions in the area cannot be managed using appropriate engineering techniques.